.alpha.-hydroxy acids are very useful compounds as precise fine chemicals such as medicines and agricultural chemicals, or intermediates thereof.
As a process for producing .alpha.-hydroxy acids, there has been known a process which comprises reacting a carbonyl compound such as an aldehyde with a cyanogenation agent such as hydrogen cyanide to produce the corresponding .alpha.-cyanohydrin compound, then hydrolyzing the obtained .alpha.-cyanohydrin compound. In this process, however, since the cyanogenation reaction is reversible, it is generally difficult to obtain the corresponding .alpha.-cyanohydrin compound (particularly, an .alpha.-cyanohydrin compound derived from an aldehyde) in high yield. Moreover, since the obtained .alpha.-cyanohydrin compound tends to decompose into the carbonyl compound and hydrogen cyanide by the hydrolysis, it is difficult to produce an .alpha.-hydroxy acid in high yield.